Coal is considered as a feedstock for carbon-based materials. Carbon nanomaterials (CNMs) have been gaining tremendous attention due to their wide range of applications in various fields. CNMs include carbon nanotubes, fullerenes, carbon dots, nanofibers, nanodiamonds, graphene nanosheets, graphene onions, etc. Among them, carbon dots (CDTs) are the newly emerging carbon nanomaterials, mainly including carbon quantum dots (CQDTs), less than 10 nm in size, graphene quantum dots (GQDTs) and graphene nanosheets with a plane size less than 100 nm. Carbon dots show many outstanding advantages such as low cost, low toxicity, robust, optical/chemical inertness, and a case of fabrication. Carbon dots have great potential application in various fields, including bio-imaging, cell-imaging, sensing, photovoltaic devices and catalysis.
Carbon-based dots have been fabricated from various carbon sources including fullerenes, glucose, graphite, graphene oxide, carbon nanosheets, biomass, and carbon fibers. Physical approaches such as lithography have also been used to fabricate graphene quantum dots from various carbon sources. However, current methods for the production of carbon dots suffer from numerous limitations. For instance, lithography techniques are expensive and impractical for the production of bulk quantities of carbon dots. Likewise, current carbon-based materials that are utilized for the production of carbon dots can be expensive in bulk quantities. Moreover, current methods of production carbon dots may not be able to control the size of the formed carbon dots. Therefore, new methods are required for the bulk production of carbon dots in a controlled manner.
Mineral acid solutions (Conc. H2SO4 and HNO3) are tested as an oxidizing agent by the facile one-step wet-chemical method to fabricate carbon dots. However, it is found that the process is explosive, time-consuming and formation of sodium salts (NaNO3) take place during the neutralization step by sodium hydroxide or ammonia solution, which is difficult to separate from the reaction media. Moreover, large quantities of water are needed instead of sodium hydroxide and ammonia solution during the neutralization step.
The related prior art references so far available in the literature for the production of carbon dots from coal are provided herein below.
Reference may be made to Nature Communication, 2013, 4:2943 (doi: 10.1038/ncomms3943) entitled “Coal as an abundant source of graphene quantum dots” which reports a facile one-step chemical method to synthesize tunable graphene quantum dots from various types of coal and coke samples. They have used mineral acids such as HNO3 and H2SO4 for producing quantum dots from bituminous coals. Detailed characterization of the produced graphene quantum dots were examined by using analytical techniques. In this work, the graphene quantum dots were synthesized from bituminous coal, coke, and anthracite coal. However, in the present invention, the method as well as the characteristics of product differs from this report with respect to the use of reagents, techniques (ultrasonication, filtration, dialysis etc.) and other product characteristics.
Reference may be made to the article in Nanoscale, 2014, 6, 7410-7415 entitled “Graphene quantum dots, graphene oxide, carbon quantum dots and graphite nanocrystals in coals”. This paper reports the preparation of single-layer graphene quantum dots (S-GODS) from six coal samples of different ranks by chemical oxidation and centrifugation methods using mineral acids (HNO3 and H2SO4). The products were characterized by TEM, AFM, XRD, Raman and FTIR. However, the process reported therein is not similar to the present method in that the cited art had used strong mineral acids in the synthesis and the product characteristics were not described in detail.
Reference may be made to ACS Appl. Mater. Interfaces, 2015, 7, 7041-7048 entitled “Bandgap Engineering of Coal-Derived Graphene Quantum Dots” in which bandgaps of photo luminescent graphene quantum dots (GQDs) synthesized from anthracite coal was engineered by the one-step chemical oxidative treatment (using HNO3 and H2SO4) at higher temperature or separation by cross-flow ultrafiltration. The products were characterized by TEM, DLS, XPS, Mass Spectroscopy, FTIR, C13 NMR, UV-visible, XRD, Photoluminescence, and Raman analysis. The average sizes of the GQDTs were found to be 4.5±1.2, 16±3.3, 41±6.4, and 70±15 nm. However, the present invention differs in the process from the stated art with respect to the use of H2O2 and ultra-sonication. They had also used inorganic acids, which is not eco-friendly. Also, the report mainly emphasizes on the band-gap engineering of the quantum dots.
Reference may be made to ACS Nano, 2014, 8 (10), 10837-10843 entitled “Boron- and Nitrogen-Doped Graphene Quantum Dots/Graphene Hybrid Nanoplatelets as Efficient Electrocatalytic for Oxygen Reduction” in which Boron- and Nitrogen-Doped Graphene Quantum Dots/Graphene Hybrid Nanoplatelets were synthesized from anthracite coal by one-step chemical oxidation method (using HNO3 and H2SO4) followed by codoping with nitrogen and boron at high-temperature annealing. The products were characterized by SEM, TEM, AFM, XPS, Raman analysis. This prior art mentioned about Boron- and Nitrogen-doped carbon dots. However, the method in the present invention is concerned with the carbon quantum dots.
Reference may be made to RSC Advances, 2014, 4 (81), 43160-43165 entitled “Chaos to order: an eco-friendly way to synthesize graphene quantum dots” in which highly ordered graphene quantum dots (GQDs) were synthesized by a rapid, simple and pollution-free method, which adopts cheap and readily available activated carbon but not coal and environmentally friendly hydrogen peroxide as raw materials through simple microwave and hydrothermal treatment. However, the present invention involves the usage of coal.
Reference may be made to Carbon, 2015, 93, 999-1007 entitled “Ethanol in aqueous hydrogen peroxide solution: Hydrothermal synthesis of highly photoluminescent carbon dots as multifunctional nanosensors” in which a novel synthetic strategy was developed for a facile, green and low-cost fabrication of highly photoluminescent carbon dots (C-dots) by hydrothermal treatment of ethanol in aqueous hydrogen peroxide (H2O2) solution. The products were characterized by TEM, XRD, Raman, XPS, FTIR, NMR, UV-visible, PL, and Time-resolved fluorescence analysis. However, the present invention adopts sono-chemical technique unlike the one reported in the art (hydrothermal treatment).
Reference may be made to U.S. patent application Ser. No. 14/888,301 which provides a method of making graphene quantum dots from a carbon source (e.g., coal and coke). The method includes exposure of the carbon source to an oxidant such as sulfuric acid (H2SO4) and nitric acid (HNO3), sonicating the carbon source in the presence of the oxidant, heating the carbon source in the presence of oxidant at a temperature of 100-150° C. for 24-48 hrs. Thus, this patent reported the use of strong inorganic acids. However, ultrasonic assisted oxidation of coal was carried out in presence of H2O2 in the present invention, which makes it simple, facile, and faster. Moreover, there isn't any detailed product characterization reported in this patent with respect to their anti-bacterial, anti-algae and cytotoxic properties.
Reference may be made to U.S. patent application Ser. No. 14/836,826 which provides a method for preparing graphene quantum dot by sonication of a carbon material consisting of graphite, charcoal by using a fenton oxidant including, hydrogen peroxide. The method includes dispersing of the carbon material in an organic solvent, such as dimethylformamide followed by oxidizing the carbon material by adding the potassium peroxymonosulfate to the carbon material, and reducing the oxidized carbon material by performing a hydrothermal reaction. The inventors included the step of sono fenton and sono photo fenton reaction of carbon material to which potassium peroxymonosulfate has been added. However, the feed materials used in the reported art were graphite and charcoal unlike the present invention which uses coal. Also, different organic solvents were used in this patent application.
Reference may be made to CN104946252 which discloses a green, pollution-free method for extracting fluorescent carbon dots from coal. The method includes ball-milling of coal and water slurry to obtain a suspension of pulverized coal black followed by its oxidation in the presence of hydrogen peroxide and removing the hydrogen peroxide by heating. Further the supernatant liquor was cooled down and freeze-dried to get the fluorescent carbon dots. It was found that the carbon dots fluorescence yield was 40-60%. They formed point carbon size distribution 1-3 cnm and crystalline with defects. Although, this patent is quite similar with respect to the reagent (H2O2) used, yet the characteristics of the claimed CDTs produced are significantly different with the above. Also, the extraction method of CDTs adopted in the present invention is different. The art reports that the point carbon has the photocatalytic properties of degradation of methylene blue and methyl orange. However, they had not reported on the significant characteristics of CTDs such as higher florescence (FL) life time (τ), higher molar absorption coefficient, higher quantum yield, anti-bacterial, anti-algae and cytotoxicity etc. Further, in the present invention the carbon source used is from low-quality high sulfur coal which is typically/chemically different and cheap from other coals. The method is advantageous over the prior art such as, less time consuming, minimum amount of water and ammonia solution is needed for the neutralization step and additional experimental steps is not required to enhance the yield of the product.
Reference may be made to Chinese Patent CN103803540 which provides a method for the preparation of graphene quantum dot from natural coal by ultrasonic treatment of mixture of pulverized coal and strong acid followed by adding hydrogen peroxide. The method includes mechanical crushing of the natural coal into different sizes powder followed by oxidation of pulverized coal with strong acid in an ultrasonic tank using Hummers method. Further, the reaction mixture was stirred overnight, centrifuged, neutralized and dialyzed to get the water soluble graphene quantum dots. However, in the present invention the carbon source used is from low-quality high sulfur coal which is typically/chemically different and cheap from other coals. The method is advantageous over the prior art such as, less time consuming, minimum amount of water and ammonia solution is needed for the neutralization step and additional experimental steps is not required to enhance the yield of the product.
Reference may be made to WO2016053411 which discloses scalable methods of producing carbon quantum dots from carbon sources with desired bandgaps by using oxidants like H2O2. However, the patent doesn't disclose anything about nature of the product characteristics such as antibacterial, antifungal, and cytotoxicity.
Reference may be made to “Multi-functional fluorescent carbon dots with antibacterial and gene delivery properties”; RSC Adv., 2015, 5, 46817-46822, DOI: 10.1039/C5RA07968C which evaluates the antibacterial activity of carbon dots on both gram positive and gram negative bacteria. However, the anticytotoxicity and antifungal characteristics of the carbon dots is nowhere reported in the prior art.
Reference may be made to patent WO 2014179708 A1 which recites a method of making graphene quantum dots from a carbon source, wherein the method comprises of exposing the carbon source to an oxidant, and the carbon source is selected from the group consisting of coal, coke and combinations thereof. Nevertheless, this reference only theoretically mentioned the usage of sub-bituminous coal and hydrogen peroxide as a carbon source and as an oxidant respectively, for the production of graphene quantum dots. But practically it was not verified. Also, it was found that the process is explosive, time-consuming and formation of sodium salts (NaNO3) take place during the neutralization step by sodium hydroxide or ammonia solution, which is difficult to separate from the reaction media. Moreover, large quantities of water are needed instead of sodium hydroxide and ammonia solution during the neutralization step. However, the present invention consumes less time, minimum amount of water and ammonia solution is needed for the neutralization step and additional experimental steps is not required to enhance the yield of the product. Further, in the present invention the carbon source used is from low-quality high sulfur coal which is typically/chemically different and cheap from other coals. The method is advantageous over the prior art such as, less time consuming, minimum amount of water and ammonia solution is needed for the neutralization step and additional experimental steps is not required to enhance the yield of the product.
Reference may be made to “Glowing Graphene Quantum Dots and Carbon Dots: Properties, Syntheses, and Biological Applications”; First published: 17 Dec. 2014; DOI: 10.1002/sm11.201402648 which accounts for the enormous potential of carbon dots in the biomedical applications (lower cytotoxicity of the carbon dots against human cells due to their excellent biocompatibility), but the carbon dots was synthesized from carbon soots, graphite, 13C and Graphite, pitch-based carbon fibres, high purity graphite rod, and polystyrene by nitric acid oxidation, electroxidation, laser-ablated, acid treatment and chemical exfoliation, electrolysis, and electrochemical method respectively, not directly from low-quality high sulfur coal, which is typically/chemically different and cheap from other coals. Further, the present invention is simple and involves wet-chemical techniques with lesser steps. The product characteristics are also very much different.
In light of the existing prior arts, it may be summarized that the use of hydrogen peroxide and ultra-sonication method are known for the production of carbon dots and carbon quantum dots from carbon source other than low-quality Indian coal which is high in sulfur content. However, all these methods used expensive and high-quality raw materials (e.g. anthracite coal, graphite, activated carbon, coke etc.); highly demanding equipment and various steps are required to get the final desired product. Compared to the prior art, the present invention is easy to control, environmental friendly, simple preparation process, uses cheap raw material (i.e. low-quality high sulfur coal), and also suitable for large scale commercial production. Furthermore, the developed fabricated product of the present invention possesses higher quantum yield, is highly water soluble, emits blue-fluorescence with higher life time, and exhibits antimicrobial and cytotoxicity activity.
Thus, keeping in view the drawbacks of the hitherto reported prior arts, the inventors of the present invention realized that there exists a dire need to provide a process for the preparation of carbon dots which uses novel ultrasonic-assisted wet-chemical method, where only hydrogen peroxides (H2O2) is used as an oxidizing agent. The method also overcomes the above-mentioned drawbacks such as it is less time consuming, minimum amount of water and ammonia solution is needed for the neutralization step and additional experimental steps are not required to enhance the yield of the product. The prepared carbon dots were characterized by Scanning Electron Microscope (SEM), High Resolution-Transmittance Electron Microscope, X-ray powder Diffractometer, Thermal analysis, Laser micro-Raman system, FT-IR spectrophotometer, UV-Visible spectrophotometer, F-2700 FL Spectrophotometer, and Time-resolved photoelectron spectroscopy.